Tree traits influence response to fire severity in the western Oregon Cascades, USA

Wildfire is an important disturbance process in western North American conifer forests. To better understand forest response to fire, we used generalized additive models to analyze tree mortality and long-term (1 to 25 years post-fire) radial growth patterns of trees that survived fire across a burn severity gradient in the western Cascades of Oregon. We also used species-specific leaf-area models derived from sapwood estimates to investigate the linkage between photosynthetic capacity and growth response. Larger trees and shade intolerant trees had a higher probability of surviving fire. Trees that survived fire tended to experience a reduction in growth immediately following fire, with the most pronounced growth suppression found in trees within stands burned at high severity. Radial growth response to fire over time differed markedly as a function of tree size. Smaller trees that survived fire generally experienced enhanced radial growth relative to small trees in unburned stands. Conversely, larger trees that survived fire experienced significant and persistent reductions in growth relative to large trees in unburned stands. There was a linear relationship between diameter and tree leaf area in stands burned at low severity, but a non-linear relationship between diameter and leaf area in stand burned at high severity. Generalized additive models are well suited to modeling non-linear mortality and growth responses to fire. This research provides a better understanding of how fire severity influences tree-growth, forest succession, as well as the long-term resilience of forests to disturbances.